Current sense transformers are used to measure current passing through a conductor. Typical applications for current sense transformers are overload sensing, load variation sensing, and electric power metering. The construction of a current sense transformer depends on the desired transformer efficiency, which in turn is dictated by the requirements of the application that uses the current sense transformer.
Current sense transformers that are required to be optimized to work at a very high efficiency use a toroidal winding wherein the secondary winding is wound around a transformer core. These toroidal winding type current sense transformers are expensive to fabricate and therefore not practical for use in applications where transformer efficiency is not critical. Laminated core transformers are less expensive to fabricate than wound core devices, and transformer windings obtained from a simple bobbin machine can be used for these laminated core transformers.
A typical bobbin-wound current sense transformer has a laminated core, a primary winding that has a single turn (or small number of turns) and a secondary winding with a large number of turns wherein the secondary winding is obtained from a simple bobbin machine. The primary and secondary windings are mounted on the core and there are separate primary and secondary terminals connected to the primary and secondary windings respectively. Insulating members separate the primary winding, secondary winding and the core from one another. Typical of this type of unit is a device manufactured by Falco Electronics. This company sells a bobbin-wound current sense transformers with a single turn primary winding and a bobbin type secondary winding. Their unit uses a round single turn primary winding which also serves as a terminal. However, the use of a round wire that passes through an insulating flange is inherently unstable.
A typical current sense transformer during operation has its primary winding connected in series with an alternating current source to be monitored. This current is coupled magnetically by the magnetic core shared by the primary and secondary windings. The current is reduced by the ratio of secondary to primary turns. A resistor is connected across the secondary winding so that the reduced current will flow through the winding and develop a voltage across the resistor. The voltage is dependent upon the value of the resistor connected across the secondary, and the amount of current flowing through the secondary.
Presently, most manufactured bobbin wound current sense transformer apparatus use a separate primary winding and a separate primary terminal which must be connected together. The primary winding is connected to one end of the terminal and the other end of the terminal is connectable to a PCB. The current apparatus requires a separate primary winding and two pins on which one end of each of the pins is used to wrap an end of the wire from the primary winding and the other ends of the pins plug into the PCB. This method/manufacture results in additional expense and complexity. The winding/terminal connection is also a potential source of problems.